The former couple's lawyer, Marty Singer, has sent Kimberly Alexander a cease and desist letter asking her to "immediately stop defaming them with malicious defamatory lies and specious fabrications" over her paternity claim.

Alexander had sent screenshots of alleged text messages between herself and Thompson to entertainment site Gossip of the City, in which she suggested he had taken a paternity test in January 2020.

Khloé Kardashian and Tristan Thompson split in February 2019. (Getty)

In the cease and desist letter obtained by E! News, Singer confirmed that Thompson did take a paternity test four months ago, but it "indisputably established" that he is not the father. However, Alexander has requested the Cleveland Cavaliers star take another test because she claimed the DNA testing facility he used had alleged links to the Kardashian family.

Singer said that Thompson has agreed to take another test, only if an accredited AABB (formerly known as the American Association of Blood Banks) lab performs the test.

"It has been proven that Mr. Thompson is not the father of your child and we demand that you stop claiming that he is, and that he and Ms. Kardashian somehow falsified the paternity test results," Singer wrote in the letter to Alexander. "It is defamatory per se to falsely assert that someone is a deadbeat dad who allegedly does not support his children, and to falsely accuse my clients of faking paternity test results."

Thompson with daughter True. (Instagram)

Singer also pointed out that Alexander was "fired" by famed US attorney Lisa Bloom after her firm was "unable to resolve your claims".

"It is obvious that the reason you are spreading these lies about my clients is because you want your 15 minutes of fame. My clients will not tolerate your despicable conduct," he wrote.

Alexander has since responded to the letter, telling E! News, "I feel like the whole issue originally stemmed from Tristan not wanting to choose a neutral setting for the DNA testing and using a Kardashian associated DNA Testing facility.

"I feel as if you really have nothing to hide then why not just go to a neutral testing site," she wrote. "And we both know what happened and exactly what went on between us, that's all I can say for now."

Thompson and Kardashian split in early 2019 but continue to co-parent their two-year-old daughter, True. The basketball star is also the father of three-year-old Prince, his son with ex-girlfriend Jordan Craig.